The invention relates to bar code reading apparatus, and particularly to a counter-type bar code scanner of very shallow depth.
Point of sale (POS) bar code scanners, such as those now commonly used in grocery stores and other retail stores, have certain basic requirements. The POS systems generally require a scanner to provide "omnidirectional" bar code read capability. In most cases scanners cannot read a label from any random orientation, but they do come very close if it is assumed that the label is moving in one general direction toward and over the scanner.
"Omnidirectional" in this context means that generally all possible label orientations in three-dimensional space can be read within some depth of field in a scan volume above the counter top or deck, assuming the packages are always coming from the same general direction. The actual field of reading generally is about 140.degree., about an axis perpendicular to the scanner top.
Almost all POS supermarket bar code scanners have conventionally used a similar scan pattern. This basic scan pattern can be defined by a series of separate apparent source locations, or locations from which a scan line appears to originate. Each apparent source can be multiplied and modified slightly, depending on the number of facets (on a mirror or a holographic deflector) used to generate a line or lines coming from the same general location. Different facets can produce scan lines which are slightly offset from one another, so these may technically be an integral multiple of the basic apparent sources. Most contemporary scanners produce scan lines based on five general apparent sources, with some variation in the pattern. These sources are termed the center horizontal, the left and right side horizontals, and the left and right verticals. Substantially all scanners currently in use employ some variation of this five apparent source pattern. Some scanners may produce four center horizontals, others may produce one center horizontal, etc., depending on the intended usage of the scanner and the skill of the designer.
The basic five apparent source scan pattern has evolved as a very efficient and elegant scan pattern for providing "omnidirectional" read capability for labels. However, several criteria well known in the art of POS scanner design have led those skilled in the art to believe it to be very difficult or impossible to create this type of scan pattern in a shallow package, such as about 12 cm in height.
One problem seen with reference to the prior art is that a vertical scan line generally must travel along a mirror inclined along the height of the package. Thus, the amount of usable vertical scan line is a function of package height. Accordingly, in the prior art a tall vertically-oriented mirror tended to be the final mirror which reflected the beam out of the scanner.
Another restriction in scanner design, relating to producing a shallow scanner, is the distance the scanning beam travels from mirror to mirror inside the scanner before it exits. This relates to the fact that a laser scanner requires a certain beam size, within certain narrow limitations, at all reading positions in the scan volume above the deck. Since the scanner contains only one set of optics for focusing the beam, which converges as it exits the scanner, all scan lines must generally have about the same distance to the counter top. The addition of more mirrors to make a scan line will increase the path length and will affect usable depth of field above the counter top. As more mirrors are added, the scanner designer must also be careful that he has not reduced mirror sizes to the extent of blocking the light collected from the bar code.
Prior art scan patterns generally were based on early work by IBM and RCA as to codes and scan line orientations that could be used to reduce sensitivity to orientation. Scanner manufacturers thus attempted to produce lines going through space which would give the best possible area coverage and volume coverage as the label was moved through the scan volume.
Some early scanners were as much as three feet high, generating a scan pattern called a stitch bar pattern, essentially an orthogonal grid. There was also a theory that a scan pattern could be configured generally as a series of orthogonally intersecting diagonals, i.e., a series of Xs. Further, according to another theory three scan lines could intersect at 60.degree. angles, in an asterisk-like pattern, for further redundancy in assuring readings.
Most of the early theory for design of scan pattern for multiple coverage of bar codes was developed in two dimensions. The bar code generally is set up in what is called an over-square condition. In this configuration the height of the bars on the label is greater than half the width of the bar pattern. Typically the ratio of the height to half the width is about 11/2:1 to 2:1. The UPC (Uniform Product Code) label is configured so that if one half of the label is read, then the other half is read, and the two readings are put together, all of the information is retrieved. Thus, a scan line need not traverse all the bars of the bar code, but only half the bars, with another scan line traversing the other half.
In designing the scan lines of a bar code scanner as seen by a label passing through a scan volume above the deck, it is necessary for "omnidirectional" read capability to have at least one scan line able to traverse half the bars of the bar code label, regardless of the orientation of the package and the label, assuming the package is moving in one direction over the scanner within predefined velocity limits. In an ideal scan geometry, no matter what orientation the package is in, the bar code label would always "see" what appear to be orthogonally crossing lines.
In a laser scanner a laser tube somewhere in the scanner package generates a beam which is routed around to a lens and then goes toward a rotating wheel. The rotating wheel may comprise a polygon, i.e., a mirror wheel with two or three mirror surfaces on a polygon which rotates, or it may comprise a hologram. In either event, the beam emerges from the rotating wheel and then it is reflected off a series of successive mirrors to exit the scanner creating a scan pattern in a scan volume above the deck of the scanner.
One concern in scanner geometry and optics design is path length. The paths of all the scan lines should generally be about the same, since the scanned laser beam is a converging one, and it has a limited depth of field wherein the spot size will be small enough in relation to a bar code label to be able to accurately distinguish the bars and spaces. Thus, if one scan line has a considerably greater path length than another, the degree of convergence of the beam in that scan line will be different from the convergence in another scan line at a given point in space, leading to excessive differences in spot size on the two scan lines.
In certain markets, particularly in European POS scanners such as in grocery stores, there is a need for a counter top scanner of considerably shallower total depth than has been available previously, to enable cashiers to sit down comfortably with their legs beneath the scanner package. This problem has been addressed in the prior art, but in ways which have generally not been acceptable. There have been some systems wherein a relatively shallow scanner height, of about 16 cm for example, has been achieved. This is generally not sufficient since a standard or usual requirement is that the scanner be less than about 13 cm deep for comfort of the cashier.
Another prior art approach has been to have a deck and associated scanner package which is of relatively shallow depth, but with a portion of the housing expending up considerably higher, and that portion including some operative components such as a holographic optical element. Still another approach has been to have the deck area of the machine quite shallow, on the order of about 10 cm, but with an obliquely upwardly extending housing portion at one side of the machine, and with the oblique portion actually having the scanned beam outlet window. This requires the cashier to orient the bar code away from himself when sitting down, making difficult the proper orientation of the bar code since it is not then seen by the cashier. It is also a more awkward approach to moving a product through a scan volume, since the scan volume is actually raised and tilted with respect to the counter top.
It is among the objects of the present invention to provide a bar code scanner of shallow profile, less than 13 cm, with five-apparent-source scan geometry or similar and with the scan volume immediately above a horizontal deck as in the typical preferred configuration, and with a high degree of reading reliability.